Fluids generally are characterized by multiple properties, for example, viscosity, density, temperature, chemical composition, phase (e.g., gas or liquid), compressibility and pressure that can be measured and determined experimentally. Determining fluid properties is important for a wide variety of industrial applications. For example, time-based maintenance of heavy equipment or machinery involves maintaining or changing fluids on a fixed time schedule. A common example of time-based maintenance involves changing the engine oil in an automobile every three months or three thousand miles, regardless of the condition of the oil to ensure that the oil is changed before its properties have degraded below minimum acceptable levels. Time-based maintenance can, therefore, lead to disposal of the oil before its useful life has ended or overuse of the oil leading to wear on the engine. Conversely, condition-based maintenance involves maintaining or changing a fluid only when certain conditions exist. For example, a fluid may be changed when the viscosity of the fluid is greater than a pre-defined value or less than a pre-defined value. Condition-based maintenance can, therefore, lead to cost savings by tailoring maintenance of a fluid to its useful life, rather than a fixed time schedule.
Acoustic wave sensing devices have been used to determine properties of a material (e.g., using surface acoustic wave or bulk acoustic wave devices). These types of devices commonly employ delay line measurements or methods. In such devices, an operating voltage is applied across input and output transducers that are coupled to (e.g., mounted on) a piezoelectric material. For example, an oscillatory operating voltage (i.e., input signal) applied to an input transducer induces a mechanical perturbation in the piezoelectric material that is measurable as an output signal by the output transducer. In delay line devices, the operating voltage can be applied for a finite time and then turned off, creating a finite oscillating wave that attenuates with time and distance as it travels. The output signal also experiences a dispersion (or smearing effect) and a time delay (with respect to the input signal) based, in part, on energy dissipation as the finite oscillating wave propagates through the piezoelectric material. Coupling a fluid to the piezoelectric material causes additional attenuation, frequency shift and time delay to occur to the oscillating wave.
Delay line measurements using acoustic devices are susceptible to reflections of the oscillating wave. More particularly, a portion of the oscillating wave generated by the input transducer interferes with a boundary of the material and is reflected back toward the input transducer. Reflection can cause interference that reduces the sensitivity of the sensor. One known method of reducing reflections and associated interference is to employ an acoustic-absorbing material in the device to absorb reflections and minimize their effect on measurement accuracy. Another drawback of certain acoustic wave devices is that they operate at relatively high frequencies (e.g., greater than 10 MHz). For example, higher operating frequencies generally necessitate more expensive electronics to observe electric signals and to observe fluid properties.
Resonators are one type of acoustic wave sensing device that actually exploits the constructive interference of oscillating wave reflections. For example, thickness shear mode (“TSM”) devices use in-plane compression waves through a material (e.g., a quartz crystal) to create a resonance condition. A quartz-crystal microbalance (“QCM”) device is one type of TSM device. QCM devices employ electrodes on opposing sides of a quartz disk such that a voltage applied across the electrodes induces a shear motion in the material. Shear motion in the material is measurable as the in-plane motion is projected onto the surfaces of the disk. The shear motion is affected by mass-loading of the surface of the device (e.g., by interacting a fluid with the surface of the device). The fluid slows or damps the shear motion in the device, which can be measured to infer fluid properties. TSM devices can operate in the relatively-low frequency range of under 10 MHz. However, a drawback of TSM devices is that their response depends on the boundary condition between the surface of the material (e.g., quartz disk) and the fluid. Because TSM devices use shear forces to couple with the fluid, conditions that reduce drag or cause slipping at the surface or interface (e.g., a surfactant or detergent additive in the fluid) hinder coupling. More particularly, a surface condition conducive to slipping reduces viscous drag on the surface of the material or reduces the effect of drag on the surface of the material, which decreases the accuracy of the measurement of fluid properties. Additionally, acoustic coupling of the TSM to the fluid results in disturbances in the fluid that travel ultrasonically (i.e., greater than the speed of sound) through the fluid. Waves propagating through a TSM device also travel ultrasonically. Waves having a wave velocity greater than the speed of sound associated with the fluid experience radiative loss from the surface that is in contact with the fluid. Radiative loss increases the complexity of measurement and reduces the accuracy of such devices.
Hence, there is a need for a method of measuring fluid properties more accurately. There is also a need for a more robust sensing device that can be used to determine fluid properties in the relatively low frequency domain or with reduced sensitivity to the boundary condition between the device and the fluid.